Direct current (dc) to alternating current (ac) inverters are finding considerable use in industrial applications whenever an ac source independent of utility power is needed. In particular, where low levels of electromagnetic noise and/or very high frequencies are required, sinusoidal inverters are needed. One example of the use of such inverters is in the field of high frequency induction heating for case hardening of machine parts. Inverters are also finding increased use in utility-interactive systems where dc generators, such as solar arrays, seek to contribute their excess power to the utility grid.
It is known in the art that a dc to ac inverter for providing a substantially sinusoidal output can be constructed by employing switches which, when open, interrupt the connection of a constant voltage source to a resonant tank circuit. Such techniques are described in the article by N. Mapham entitled "An SCR Inverter with Good Regulation and Since Wave Output", IEEE Transactions on Industry and General Applications, Vol. IGA-3, No. 2, March/April 1967, which article describes such a voltage mode, or series, inverter.
The voltage mode inverter has certain unsatisfactory characteristics. For example, the most effective configurations for voltage mode inverters, such as that referenced above, require that the switches carry current in both directions. Hence, a fast, and a relatively expensive, diode must be employed in parallel with each switch, the diode being arranged to carry the reverse current. Moreover, damping circuits in parallel with each switch are usually necessary to suppress transients and to limit the rate of rise of voltage across an opening switch. Additionally, the common half-bridge voltage mode circuit requires a center-tapped voltage source, usually approximated by two large, and relatively expensive capacitors connected in series and placed in parallel with a single voltage source. It should also be noted that, when silicon controlled rectifiers (SCRs) are used as switches in such voltage mode inverters, the frequency of the sinusoidal output is limited to an upper range of about 30 kHz.
Thus, there exists a need for a less expensive sinewave inverter which uses components which are less susceptible to damage or failure and which is capable of performing in a wide variety of industrial applications and, particularly, in high frequency applications.